Contaminated milk has poisoned 54,000 children in China at last count, according to officials.

Nearly 13,000 remain in hospital after drinking milk products that contained the industrial chemical melamine. Four children have died and more than 100 are seriously ill.

Amid growing public outcry, Wen Jiabao, the Chinese premier, visited hospitals in Beijing over the weekend and threatened harsh punishment for the culprits.

"The public is worried, doctors are worried, and we're also worried," he said, according to the state-run Xinhua news agency.

"What we are trying to do is to ensure no such event happens in future, by punishing those responsible. None of those companies lacking professional ethics or social morals will be let off.

"The most crucial point is that after a clean-up there can be no problems at all with newly produced milk products. If there are fresh problems, they must be even more sternly punished under the law."

Japan, Brunei, Singapore, Malaysia and Taiwan are among Asian countries that have banned or recalled Chinese milk products. Melamine was found in products from China's three biggest dairy companies.

The chemical is used to make plastics and is high in nitrogen, which makes products appear to have a high protein count. Ingesting small amounts is not thought to cause harm, but sustained consumption can cause kidney stones and renal failure, especially in children. Most of the victims are under two years old.

Hong Kong is thought to have discovered its first victim: a three-year-old girl diagnosed with a kidney stone at the weekend. She had drunk two glasses a day of a high-calcium, low-fat milk product for more than a year. Other children in the territory are being tested.

The Sanlu Group, China's biggest infant milk powder producer, knew of the problems with melamine for more than a month but kept quiet while Beijing hosted the Olympics.

The government has promised free treatment for affected children, but parents are worried about long-term illness and doctors' bills.

The scandal is hitting the farming industry. The Chinese ministry of agriculture said despairing farmers were dumping milk and killing their cattle after companies stopped buying their supplies. The ministry has promised subsidies to prop them up.

The milk scandal has again shaken trust in Chinese products after scares last year over toxic and shoddy goods ranging from toothpaste and drugs to pet food and toys.